Many mechanical, electrical and electronic devices incorporate one or more mechanical or electro-mechanical resonator to achieve their functionality and response. A first class of such devices include clock generators, reference frequency generators, crystal oscillators, RF or microwave oscillator, local frequency oscillators, etc. in which the resonator sets (or contributes to set) the frequency of the device's output signal. We refer to this first class of devices as clock generators. A second class of such devices include filters (including radio frequency (RF) filters, etc.), duplexers (and similar devices that feature more than 2 ports, e.g. quadplexers), circulators, isolators, diplexers (and similar devices that feature more than 2 ports, e.g. triplexers, etc.), combiners, splitters, waveguide, etc. in which the resonator(s) set (or contribute to set) the frequency response of the device. We refer to this second class of devices as filters. A third class of such devices include sensors (including acceleration sensors, vibration sensor, pressure sensors, etc.), actuators (such as ultrasound wave generators, force actuators, momentum actuators), electrically-controlled mirror, electrically-controlled pump, other types of electro-mechanical transducers, etc. in which the resonator sets (or contributes to set) the conversion factor (also referred to as sensitivity, gauge factor, etc.) of the device. We refer to this third class of devices as transducers.
In order to achieve their targeted performance, many of the above-mentioned clock generators, filters and transducers require one or more constituent resonator(s) to operate at a fixed or near-fixed frequency (or a metric related to the resonator response, as discussed below) over a specified temperature range. The deviation from such a specified frequency is referred to as frequency drift. A resonator that features little or low frequency drift is regarded as having a high degree of frequency stability. Resonators are not required to have a high degree of frequency stability. For other applications, the resonator frequency is required to change slightly as a function of temperature. For example, in certain clock generator designs, the oscillator circuit causes the oscillation frequency to change over temperature, and the resonator can be specified to have a slight frequency drift in order to cancel the circuit-induced frequency drift. Described herein are methods and devices that control the temperature-induced resonator frequency drift to achieve zero drift as well as to achieve a non-zero drift within the range of +/−5 ppm per degree Kelvin.
Frequency drift, typically, is defined as the change of frequency at which the real part or the imaginary part or the magnitude or the argument (i.e. phase) of the resonator's complex impedance equals a specified value. Changes in frequency associated with a minimum or maximum or with a specified response criterion or set of criteria are other examples of frequency drift. Changes in frequency associated with any element of the resonator's impedance matrix or any related metric such as the scattering matrix or any other stimulus and/or response metric including non-linear metrics (such as so-called “X parameters”), time-variant metrics, etc. or any combination thereof) are yet further examples of frequency drift. Alternately, frequency drift can be defined as the change in the range of frequency (or bandwidth) over which a response criterion is met, such as the change in the bandwidth in which a resonator's insertion loss is higher than −1 dB. However, one skilled in the art will appreciate that metrics based on the resonator response other than frequency can be relevant for different applications. For example, for a RF filter application, a drift in device performance can result from the change of insertion due to temperature change. For purposes of illustration herein, the description of the invention is framed in the context of frequency drift but the control of frequency drift described herein also applies to more general resonator response drift metrics.
Frequency drift over temperature is typically expressed or specified in part-per-million per degree Kelvin (ppm/K), or in part-per-million over the specified operating temperature range (or, alternately over a given ambient temperature range, etc.).